Histone-histone and histone-DNA interactions are involved in the formation of the basic chromosomal unit, the nucleosome. The histones are heterogeneous, with each class consisting of a number of non-allelic variants. The ratio of these variants in different cell types may be a specific characteristic required for optimal genomic functioning. Recent studies have shown that the nucleosome repeat length varies between cell types of different transcriptive activities. The nucleosome repeat lengths may be a basic organizational element in chromatin, variations in which affect both structure and activity of the chromatin. Histone variants may be involved in determining the repeat lengths of different cell types. My studies plan to analyze the nucleosome repeat lengths during erythroid development in normal mouse cells and those transformed by the Friend leukemia virus. I shall also examine the possible role histone variants may have in affecting the nucleosome repeat lengths of these cell types. Histones from staged normal erythroid cells and Friend erythroleukemic cells will be analyzed and compared by iso-electric focussing in polyacrylamide gels. A time course analysis of the nuclease digest of nuclei from these stage cells on agarose-polyacrylamide slab gels will reveal if the oligomer repeats from the different stages are not explained by multiples of a common single unit. Such a finding would indicate a difference in the nucleosome repeat lengths. As one approach to linking histones or non-histones to the alteration of the nucleosome repeat lengths of different cells, reconstitution experiments are planned using histones and non-histones from cells with different repeat lengths. Since histone variants are conserved through evolution, these studies aim to show what their function may be in effecting changes in chromatin structure.